Existing commercial transport jet aircraft typically include two or more primary turbine engines for propulsion. These aircraft also typically include at least one auxiliary power unit (APU) that provides power in addition to or in lieu of the power provided by the primary engines. Accordingly, APUs can be used to provide power to the aircraft when the primary engines are not running, for example, while the aircraft is waiting at an airport gate. The APUs can also provide temporary power to start the primary engines during normal operations, and/or temporary emergency power during an engine-out condition or other emergency condition.
Over the course of time, aircraft manufacturers and airlines have come under increasing pressure to reduce the noise emitted by aircraft during normal operations, including gate operations, taxi, takeoff, and landing. Accordingly, aircraft manufacturers have developed mufflers and other sound attenuating devices to reduce the noise emitted by both the aircraft primary engines and the APUs. One drawback associated with APU mufflers is that they can add a significant amount of weight to the aircraft, necessitating an increase in aircraft structural capabilities and reducing overall aircraft performance. In particular, long mufflers may provide adequate noise attenuation, but may unacceptably reduce overall aircraft performance because they increase the aircraft weight and drag without increasing payload capacity. Short mufflers, while having less of an adverse impact on the aircraft performance, may not comply with guidelines and standards that have been driving acceptable aircraft noise levels downward. Aircraft designers are accordingly confronted with conflicting demands by airlines that include requests for increased aircraft performance on the one hand, and requests for reduced aircraft noise on the other.